The present invention relates to electronics circuits in communications systems, and more particularly to transmitter architectures that provide improved performance.
The design of a high performance transmitter is made challenging by various design considerations. For many applications, high performance is required to meet system specifications. High performance can be characterized by the linearity of the transmit signal path, a wide dynamic range to control the transmit power, and other characteristics. Moreover, for some applications such as cellular communications systems, power consumption is an important consideration because of the portable nature of the cellular telephones. Cost is also a major consideration for many transmitter designs that are incorporated into mass-produced consumer products. High performance, low power consumption, and low costs are generally conflicting design considerations.
These various design considerations effect the performance and acceptance of many consumer products such as, for example, cellular telephones. Examples of cellular communications systems include Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), and analog frequency modulation (FM) communications systems. CDMA communications systems are disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM
USING SATELLITE OR TERRESTRIAL REPEATERS,xe2x80x9d and U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d both assigned to the assignee of the present invention and incorporated herein by reference. CDMA communications systems are also defined by xe2x80x9cTIA/EIA/IS-95-A Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d and xe2x80x9cTIA/EIA/IS-95-B Mobile Stationxe2x80x94Base Station Compatibility Standard for Wideband Spread Spectrum Cellular System,xe2x80x9d both of which are incorporated herein by reference.
In CDMA communications systems, nonlinearity in the transmitter generates intermodulation distortion that acts as noise and degrades system performance. To reduce nonlinearity, the elements in the transmit signal path are designed to operate in their linear regions and, as the result, consume large amounts of power. Wide dynamic range is required to adequately control the output transmit power. In CDMA systems, the transmit power level is adjusted to provide the required system performance (i.e., a particular bit error rate), low interference to other units, and reduced power consumption. Low power consumption by the transmitter allows for the use of a smaller sized battery, which often translates to a smaller sized telephone. Smaller size is highly desirable because of the portable nature of the telephone. Low power consumption by the transmitter further allows for increased talk and standby times for a specified battery size.
As can be seen, transmitter architectures that provide high performance, low power consumption, and low costs are highly desirable.
The invention provides controller circuits that govern the operation of transmitters for a communications system to provide improved performance over conventional transmitters. The improvements include a combination of the following: faster response time for the control signals, improved linearity in the output power adjustment, reduced interference, reduced power consumption, lower circuit complexity, and lower costs. For a cellular application, these improvements can lead to increased system capacity, smaller telephone size, increased talk and standby times, and greater public acceptance of the products.
An aspect of the invention provides a transmitter in a communications system that includes a variable gain element, a power amplifier section, and a controller circuit. The variable gain element has a variable gain covering a particular gain range. The power amplifier section couples to the variable gain element and includes a number of discrete gain settings, with one of the gain settings being a bypass setting. The controller circuit provides the control signals for the variable gain element and the power amplifier section. The gains of the variable gain element and the power amplifier section are updated in a manner to reduce transients in the output transmit power and to provide linear adjustment of the output transmit power level. The variable gain element and the power amplifier section are also controlled to reduce power consumption, e.g., by powering down one or more sections when not needed.
Another aspect of the invention provides a method and apparatus for adjusting a gain of a circuit element in a transmitter. In accordance with this method and apparatus, a gain control signal that includes gain setting values for the circuit element is received. Overdrive pulses corresponding to changes in the gain setting values are then generated. The overdrive pulses are summed with the gain setting values to generate an adjusted control signal, which is filtered to generate a filtered control signal. The gain of the circuit element is then adjusted in accordance with the filtered control signal. The overdrive pulses can have amplitudes that are related to the magnitude of the changes in the gain setting values and can also have programmable duration.
Another aspect of the invention provides a method and apparatus for adjusting signal gain in a transmitter having a first gain element and a second gain element. The first gain element responds to a first update clock and the second gain element responds to a second update clock. The first and second update clocks are asynchronous. In accordance with this method and apparatus, the first and second gain transfer characteristics of the first and second gain elements, respectively, are determined. A gain compensation table is then generated based on the first and second gain transfer characteristics. During normal operation, first and second gain setting values for the first and second gain elements, respectively, are received. The second gain setting value is adjusted with a particular gain offset value based on the first gain setting value. A linearized gain setting value corresponding to the adjusted second gain setting value is then retrieved from the gain compensation table. The gains of the first and second gain elements are adjusted with first and linearized gain setting values, respectively.
Another aspect of the invention provides a method and apparatus for adjusting signal gain in a transmitter having a first gain element and a second gain element. The first gain element responds to a first update clock and the second gain element responds to a second update clock. The second update clock is faster than the first update clock and the first and second update clocks are asynchronous. In accordance with the method and apparatus, the first and second gain setting values are received for the first and second gain elements, respectively. The first and second gain control signals representative of the first and second gain setting values, respectively, are then generated. The first and second gain control signals are aligned with the first and second update clocks, respectively. Changes in the gain setting value of the first gain element are detected. If a change in the gain setting value is detected, the second gain control signal is aligned with the first update clock; and if no change in the gain setting value is detected, the second gain control signal is aligned with the second update clock. The gains of the first and second gain elements are adjusted with the aligned first and second gain control signals, respectively.
Another aspect of the invention provides a method and apparatus for providing linear adjustment of output power level from a transmitter. The transmitter includes an element having a number of discrete gain settings and an element having a continuously variable gain setting. In accordance with the method and apparatus, a gain transfer function of the transmitter is determined for each of the discrete gain settings. For each of the discrete gain settings, a gain compensation table is generated based on the determined gain transfer function. A first gain setting value for the element having discrete gain settings is received. The first gain setting value identifies one of the discrete gain settings. A second gain setting value for the element having variable gain setting is also received. A compensated gain setting value is retrieved from the gain compensation table corresponding to discrete gain setting identified by the first gain setting value. The gain of the element having discrete gain settings is adjusted with the first gain setting value, and the gain of the element having the variable gain setting is adjusted with the compensated gain setting value.
Another aspect of the invention provides a method and apparatus for controlling transients in the output power of a transmitter during a signal transmission. The transmitter includes a first element having a first time response and a second gain element having a second time response. The first time response is faster than the second time response. In accordance with the method and apparatus, first and second commands are received to adjust the gains of the first and second elements, respectively. The first command is delayed by a particular time period. The gains of the first and second gain elements are adjusted with delayed first command and the second command, respectively. The particular time period is selected to reduce increase in output power level of the transmitter due to adjustment of the gains of the first and second elements. In an embodiment, the first command is delayed when an increase in the gain of the first element is detected.
Another aspect of the invention provides a method and apparatus for controlling a power amplifier in a transmitter during a signal transmission. In accordance with the method and apparatus, the required output transmit power level is first determined. If the required output transmit power level is below a particular threshold, the power amplifier is bypassed and powered down. If the required output transmit power level exceeds the particular threshold, the power amplifier is powered up for at least a particular warm up period and then selected for use. The power amplifier can be powered down when not in use. The selection and bypassing/powering down of the power amplifier can be performed at times corresponding to boundaries of transmitted code symbols to minimize degradation in system performance. In a similar manner, the transmit signal path (e.g., the transmit RF and IF chain), as well as the biasing circuitry, can be powered down when not in use.